MIAMI, Florida (CNN) -- Something fishy's going on in Miami, and police and Florida wildlife officials said they're trying to get to the bottom of it.

After a short ride on Miami's elevated Metromover train, a dead shark turned up in the middle of the street, in the middle of the city, in the middle of the night, police said.

Police were called about 9 p.m. Tuesday and found the 5-foot-long nurse shark lying on Fifth Avenue in Miami's Overtown section. How the shark got there -- and why -- is the subject of an ongoing investigation.

The mystery began Sunday when a Metromover passenger took a photo of the shark, bleeding and gasping, as it lay on the floor of the train. The passenger sent the photo to CNN affiliate WSVN-TV in Miami-Fort Lauderdale.

The shark turned up dead on the street, covered with flies, on Tuesday.

"I have never seen a shark in the middle of the street in Overtown, but nothing surprises me in Miami," said Jorge Pino of the Florida Fish and Wildlife Conservation Commission, which is conducting the investigation.

Police and wildlife officials said two men, whom they described as vagrants, took the shark on the Metromover after failing to sell it at several fish markets.

David Gil, who works at the Casablanca Fish Market, said the men approached him.

"Two guys pulled up on a bicycle; they were both riding and were dragging the shark," Gil told CNN. "They said it was like 16 feet and about 600 pounds, but when I went out to take a look at it, it was tiny. I'd say it was about 5 feet and about 75 pounds."

He added, "They had it on a rope with a stick tied to its tail. They were dragging the shark all over Miami, and they were telling me they were trying to sell it for 10 bucks."

The men were turned away because they did not have a permit to sell the shark, said Martha Longueira, who handles purchasing for the Casablanca market.

"We do get fish peddlers, and people assume that if they go out fishing, and have 20 pounds of fish, they can just sell it ... but we can't accept it," she said.

Witnesses told police the men dragged the shark down the street.

"They were pulling cars over to see if anybody wanted to buy it, then they just left it in the street," Gil said.

Miami police have turned the investigation over to the Florida Fish and Wildlife Conservation Commission.

Nurse sharks are not protected species, so it is not against the law to possess one, Pino said. Investigators are looking into whether the men were selling the shark without a license or caused willful and wanton waste.

"Because it was discarded, they just chose to drag it into the middle of the street, and it caused havoc for everybody," Pino said

The shark's remains were returned to the ocean, he added

"We returned it back to its natural habitat. If a fish in the wild dies, its consumed by other fish," Pino said. "After we gathered our information, we returned it to the water, off the coast of Miami."